Physical image vs. Structure relation. Part 3. Basic properties and protonation mechanism of some tetraaza macrocyclic ligands

The protonation constants, log K, for 1,4,7,11-tetraazacyclotetradecane (is ocyclam, 2), 1-(2-aminoethyl)-1,4,8,11-tetraazacyclotetradecane (scorpiand, 3), 5,12-dimethyl-1,4,8,11-tetraazacyclotetradecane (Me(2)cyclam, 4) and 5 ,5,7,12,14,14-hexamethyl-1,4,8,11- tetraazacyclotetradecane (Me(6)cyclam, 5 ) were determined pH-metrically. Attempts of correlation of the calculated enthalpy of protonation in the gas phase (AM1 method) with experimental val ues of the protonation constants for ligands 1, 2, 4-7 were done 1,4,8,11-t etraazacyclotetradecane, cyclam, 1; 1,4,7,10-tetraazacyclotetradecane, cycl en, 6; 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane, (N-Me)(4)cyc lam, 7. Extensive NMR pH-titrations, i.e., determination of pH vs. chemical shifts (H-1 and/or C-13) plots, (delta(X) = f(pH), allowed to suggest the most likely protonation schemes of all nitrogen atoms in the cyclic polyami nes 1-3. The possibility of the formation-breaking of the intramolecular hy drogen bonds, as well as the change of conformation of these polybasic macr ocycles during protonation-deprotonation steps, has been considered on the basis of the supplementary theoretical calculations (MMX/STO-3G study).